1. Technical Field
The present invention relates mainly to transmissions for vehicles.
2. Related Art
Conventional transmissions include a transmission disclosed in Japanese Unexamined Patent Application Publication (Translation of PCT Application) (JP-T) No. 2009-536713. This transmission includes a low gear and a high gear that are attached to an output shaft so as to freely rotate, a hub fixed to the shaft between the low gear and the high gear, and a first key and a second key that are attached to the hub so as to move freely in an axial direction and rotate integrally therewith in a circumferential direction.
According to this transmission, when the first key and the second key are moved to the low gear by an actuator during acceleration, for example, the first key engages with a dog provided on a side face of the low gear such that power transmission between the low gear and the hub is realized by the first key alone. At this time, the second key is disengaged from the low gear and can therefore be moved to the high gear while power transmission via the first key is underway.
When the second key is moved to the high gear, the second key engages with a dog provided on a side face of the high gear such that power transmission between the high gear and the hub is realized by the second key. When a power transmission path is switched from the low gear to the high gear, a rotation speed of the shaft decreases, and therefore the engagement between the first key and the low gear is released at the same time as the switch in the power transmission path so that the first key can be switched to the high gear. By moving the first key to the high gear, a gear shift (an upshift) from the low gear to the high gear can be completed without causing torque interruption.
In the transmission described above, however, each of the keys is engaged with the corresponding gear in a condition where a rotation speed difference exists between the key and the gear (to be referred to hereafter as “differential rotation”), and therefore, when the key engages with the dog of the gear, torque variation (to be referred to hereafter as “spike torque”) occurs in which the torque jumps momentarily and then returns to normal. When spike torque is generated during a gear shift in this manner, an impact sound is generated by the engagement between the key and the dog, noise is generated when an outer race of a bearing that supports the shaft impinges on a transmission case, and so on. Moreover, the spike torque may generate torsion in the shaft, which causes vibration in a drive wheel and the transmission case.
Hence, a transmission that suppresses noise and vibration by incorporating a damping mechanism constituted by an elastic body in gears in order to absorb the spike torque, such as that disclosed in JP-T No. 2010-510464, has been proposed.
In the transmission disclosed in JP-T No. 2010-510464, however, noise and vibration are not suppressed efficiently, and therefore vibration is generated in a vehicle in which the transmission is installed, causing discomfort to a passenger of the vehicle. Further, the damping mechanism is incorporated into all gears, leading to increases in the cost and size of the transmission. Moreover, incorporating the damping mechanism into the gears leads to a reduction in a rigidity of the gears: particularly when the damping mechanism is incorporated into a gear having a small diameter, it is impossible to secure a sufficient damping function.